1. Field of the Invention:
This invention relates to photographic apparatus such as video cameras or still cameras, and more particularly to a drive device for the photographic lens.
2. Description of the Related Art:
It is well known that there has recently been developed photographic apparatus having a function of automatically driving the photographic lens, or the auto-focusing function or power zoom function.
In the auto-focusing apparatus, for example, a new mode is provided for measuring not only finite object distances but also infinite one. For the purpose of treating information representing the latter, use is made of a microcomputer.
Meanwhile, getting a camera with such an auto-focusing apparatus, the user who has already had the visual-focus camera with a wide variety of lenses therefor is obliged to buy more lenses of equivalent focal lengths thereto but in mountings of a new type that has a so-called "infinity sensor" switch for detecting whether or not the focusing member has reached that terminal end of movement which focuses the lens at infinity, because, if this switch is absent, the motor as the drive source for the focusing member continues being energized even after the photographic lens has been focused on an infinitely distant object. This will result in too much premature consumption of the battery or serious damage of the control circuit for the motor. Hence, there is a demand for making it possible that the apparatus operates with even the photographic lens in the old mounting having no such "infinity sensor" switch without causing the above described problem when the object distance is at infinity. Though what has been discussed here has been in connection with the focusing, the same will hold even in the case of zooming.
To control the speed of movement of the focusing member or the zoom members, there are two main methods one of which is to vary the voltage applied to the motor, and the other of which is to vary the pulse width or the so-called duty ratio. From the standpoint of minimizing the bulk and size of the apparatus itself, it is desirable to employ the latter method. In the prior art, however, it has been the common practice that the motor control circuit is provided with an additional oscillator or timer circuit solely used for determining the duty ratio, because it is on the basis of its period that determination of the pulse width is made.